FIG. 23 is a circuit diagram of conventional angular velocity sensor 5003 described in Patent Literature 1.
Sensor element 201 having an H-shape includes driving electrode 202, monitor electrode 203 and sensor electrode 204. A signal is input to driving electrode 202 to vibrate sensor element 201. Monitor electrode 203 outputs an electric charge responsive to amplitude of the vibration. When an angular velocity is applied to sensor element 201, sensor electrode 204 outputs an electric charge corresponding to a Coriolis force. A monitor signal output from monitor electrode 203 is input to drive circuit 205. Drive circuit 205 outputs to driving electrode 202 a driving signal adjusted according to the input monitor signal as to vibrate sensor element 201 with constant amplitude. A clock signal output from drive circuit 205 is supplied to timing control circuit 206 and sensor circuit 207. Timing control circuit 206 includes a PLL circuit. A sensor signal output from sensor electrode 204 is input to sensor circuit 207. Sensor circuit 207 detects the sensor signal output from sensor electrode 204 with a signal synchronized with a driving frequency of the sensor element output from drive circuit 205, and outputs an angular velocity signal corresponding to the angular velocity.
An operation of conventional angular velocity sensor 5003 will be described below.
When an alternating-current (AC) voltage is applied to driving electrode 202, sensor element 201 vibrates in a direction of an X-axis at the driving frequency. When an angular velocity is applied to sensor element 201 about a Z-axis, sensor element 201 vibrates in a direction of a Y-axis at a detecting frequency due to a Coriolis force. Sensor electrode 204 outputs a signal corresponding to an electric charge generated by this vibration, and a sensor circuit processes and outputs this signal to detect the angular velocity.
FIG. 24 is a block diagram of another conventional angular velocity sensor 5004 including digital drive circuit 208 implemented by digital circuits. Digital drive circuit 208 performs digital signal processing. Digital drive circuit 208 samples a monitor signal output from monitor electrode 203 with a clock signal of a fixed frequency output from oscillation circuit 209. In addition, digital drive circuit 208 performs digital signal processing to the sampled signal, and outputs to driving electrode 202 a driving signal adjusted to drive sensor element 201 to vibrate with constant amplitude. Digital drive circuit 208 outputs a multi-bit signal. This signal is input to timing control circuit 206 including a PLL circuit. Timing control circuit 206 outputs a detecting-phase timing signal. The detecting-phase timing signal is input to sensor circuit 207. Sensor circuit 207 performs synchronous detection with the detecting-phase timing signal, and outputs an angular velocity signal.
Digital drive circuit 208 produces jitter noise having periodicity as a phase error of the detecting-phase timing signal since digital drive circuit 208 operates based on the clock signal of fixed frequency output from oscillation circuit 209. This causes a periodical fluctuation of the output signal of sensor circuit 207, and tends to produce fluctuations of the signal output from sensor circuit 207.
The PLL circuit in timing control circuit 206 produces an output after reducing the jitter noise existing as a phase error by multiplying the multi-bit signal and integrating the jitter noise in time domain. In other words, the PLL circuit exhibits a characteristic of a low-pass filter as a frequency characteristic of an input to output phase response that indicates how a phase of the output signal responds to changes in phase of the input signal.
In conventional angular velocity sensor 5003, an undesired signal of the same phase as the monitor signal is removed from sensor electrode 204 even when there is an unbalance of the mass of sensor element 201 since it performs synchronous detection with the driving frequency of sensor element 201.
However, when the detecting-phase timing signal has jitter, a phase shift occurs according to the jitter in the detecting operation of sensor circuit 207. Due to the phase shift, the undesired signal that needs to be removed by the synchronous detection leaks to the output by as much as the product of the undesired signal and the phase shift. This results in occurrence of noise in the sensor output, hence preventing an accurate detection of the angular velocity.
FIG. 25 is a circuit diagram of still another conventional angular velocity sensor 5005 disclosed in Patent Literature 2.
Sensor element 401 made of a silicon material includes driving electrode 402, monitor electrode 403, and sensor electrode 404. A signal is input to driving electrode 402 to vibrate sensor element 401. Monitor electrode 403 outputs a monitor signal responsive to amplitude of the vibration of sensor element 401. Sensor electrode 404 outputs a sensor signal corresponding to a Coriolis force produced by an angular velocity applied to sensor element 401.
A monitor signal output from monitor electrode 403 is input to drive circuit 405. Drive circuit 405 outputs to driving electrode 402 a driving signal adjusted based on the input monitor signal to cause sensor element 401 to vibrate at constant amplitude. A sensor signal output from sensor electrode 404 is input to sensor circuit 407. Synchronous detection circuit 408 of sensor circuit 407 performs synchronous detection of the sensor signal output from sensor electrode 404 with using a signal synchronized with a driving frequency of sensor element 401, and outputs an angular velocity signal corresponding to the angular velocity. Memory 409 is implemented by a ROM. Temperature sensor 410 measures an ambient temperature. Output adjusting circuit 411 corrects an output signal detected synchronously by sensor circuit 407 based on data stored in memory 409.
An operation of conventional angular velocity sensor 5005 will be described below.
When an AC voltage is applied to driving electrode 402, sensor element 401 vibrates in a direction of an X-axis at the driving frequency. When an angular velocity is applied to vibrating sensor element 401 about a Z-axis, sensor element 401 vibrates in a direction of a Y-axis at a frequency of detection due to a Coriolis force. This vibration causes a change in capacitance of sensor electrode 404. Sensor circuit 407 executes C-V conversion of this change in the capacitance and outputs it as a voltage for detection of the angular velocity.
FIGS. 26A to 26D show relations between voltages in conventional angular velocity sensor 5004 and ambient temperatures around angular velocity sensor 5004. An operation output adjusting circuit 411 in response to a voltage output from sensor circuit 407 changing linearly with respect to the change in the temperature as shown in FIG. 26A will be explained below.
FIG. 26B shows the relation between the temperature and a voltage output from temperature sensor 410. FIG. 26C shows voltage output from memory 409 in response to the output from temperature sensor 410. Memory 409 stores correction data that indicates the relationship between the temperature and the voltage. Output adjusting circuit 411 corrects the signal output from sensor circuit 407 with using a voltage output from memory 409 according to the temperature. FIG. 26D shows the signal corrected by output adjusting circuit 411. The signal corrected by output adjusting circuit 411 does no change according to the temperature.
In conventional angular velocity sensor 5004, the intersection of the voltage shown in FIG. 26A and the voltage shown in FIG. 26C changes due to a change of the intercept on the X-axis of the voltage output from temperature sensor 410 shown in FIG. 26B, accordingly producing an offset in the corrected output signal shown in FIG. 26D.